DEVELOPMENT OF A SOLID MODEL/FINITE ELEMENT MODEL

OF A TYPE 23 FRIGATE

Answers to Interview Questions

Made by: Sulaiman Dawood Barry

Submission Date: 25th January 2010

Question 1:

Consider the Type 23 frigate; what are five most important

features from creating a computer based model of a ship.

Explain your choice.

Answer:

In order for the ship to float, the weight of the ship must be

equal to the displaced volume of the sea water. Since a warship

is made of steel, the density of which is greater than water,

therefore a solid ship would have greater weight than displaced

water volume and hence it would not be able to float. To

counter this, the ship’s hull is made hollow. And beams are

used to support the load exerted by water and the structure

above it.

The center of gravity (CoG) should be in the midplane,

therefore the ship is made to be symmetrical about the it.

Otherwise, the center of floatation (buoyancy) might not be in

line with CG, thus giving it a reason to be overturned.

The length to breath ratio is kept large so that it resembles

frigate and it is also made streamlined in order to reduce the

drag hence resulting in an increased speed, fuel efficiency and

better maneuvering than oil tankers which smaller length to

breadth ratio.

The hull is covered from the top and made water tight, so that

no water seeps inside.

Question 2:

Develop a simple Solid Model of type 23 Frigate using Pro/E or

SolidWorks, reflecting your answer to Question 1. Describe the

steps taken during the creation of the model.

Answer 2:

STEPS TAKEN DURING MODELING OF HULL

First the top plane is selected as the water plane parallel to it 4

planes are drawn.

1. The 1st Plane DTM1 is made 2 units away from the top

plane,

2. DTM4 is also 10 units far from DTM1,

3. DTM2 is 40 units from DTM1, and

4. DTM3 is 40 units away from DTM2

On each water plane, waterlines are drawn that are assumed to

be similar to Type 23 frigate. The water line drawn at the top

plane which resembles the submerged portion of the hull, it is

drawn as smooth as possible using lines and tangent arcs so

that it forms a continuous profile with no roughness in

between. Thus making it streamlined. The final shape looks like

the following.

Figure 1: Sketched portion on the top plane

Now by using the use command in sketch mode, the sketch

from the top plane is imported in each plane and then it is

undergoes changes to give the final result as shown in the

diagram below.

Figure 2: Isometric view of final Sketches of the hull

Figure 3: Orthographic view of final Sketches of the hull

To make the solid hull from swept Blend, we perform the

following steps

Sketch Tool Select Front Plane Click Sketch button in the

reference window

This enters into the sketch mode. Now perform the following

steps to make the reference planes for the swept

Select Sketch from the menu bar References Select two

extreme planes, i.e. the Top Plane and DTM3 Click solve

command in the references dialogue box.

After this step a straight line is drawn to connect these planes.

We then exit the sketch mode.

From Insert menu in the Menu Bar Swept Blend On the

dashboard click Sections Selected Sections

Select Top Section and Click Insert, it becomes orange in color.

Repeat this step for each section, from top to bottom with

sequence

After this accept by clicking the green tick mark and you will get

the following image:

Figure 4: Solid Model of Half Hull

To complete the solid hull, select the ship and then mirror it

about the front plane using mirror .

Figure 5: Symmetric Model of Hull

Using the shell tool , remove the material from the inside and

we now only we have to add ribs to the inside of the hull.

Figure 6: Hollow Hull

To add the ribs, we have to perform the following tasks

Make parallel planes on at the location at which the ribs are

supposed to be added using Datum Plane option. Using the Rib

Tool , click sketcher button and then click on to the plane on

which the ribs are to be added.

Main Menu Sketch Reference Select the cursor with a

an X mark Select wireframe then on the drawing

board click hull to make it reference line of the hull, also select

the two other planes to completely close the rib. Click solve

button to solve it the reference planes and the select a line and

make it diagonal as show in the diagram to make the rib.

Figure 7: Making Rib Model

On the dashboard click check button to exit and make sure that

the direction of the arrow is down to make it solid. Finally

specify the thickness of the rib.

Figure 8: First Rib of the Hull

Follow the same procedure to add the other rib and then

mirror both the ribs about the midplane to make it on both

sides.

Figure 9: Ship’s Hull Complete form

This Completes the Hull Part of the body

MODELING OF DECK

Each part is first made as half plane then mirrored as in the

previous sections

The sketch of the top-half plane is copied and pasted in the

sketcher mode of the new part file. The sketch is extruded to

some about thickness of 10 units .

The helipad’s half-sketch is shown and then selecting the

material removal option from the extrude menu. It is then

extruded 5 units below.

Figure 10: Helipad

Radar Tower is made using blend protusion with smooth option

selected. A rectangle is drawn and then right click and select

the toggle section option. Make a smaller rectangle centered at

the top portion of the previous rectangle. Your screen should

look then like this.

Select the check option to exit the sketch mode and enter the

extrusion depth of 100 units.

The cannon is made using revolve protusion option. First a

sketch is made ahead of the radar tower as shown and then it is

given a revolved protusion of 180 degrees.

The gun portion which is at an angle is made by selecting the

plane of the tower and moving it near the cannon. A small

rectangular section is made and then extruded to complete the

canon.

Finally the half-deck is mirrored to complete it.

Question 3:

Using the model develop for question 2, highlight the regions

that you would consider to be of interest when developing a

finite element model of the vessel. These may be due to

geometrical difficulties when meshing, stress concentrations,

mesh refinement vs element budget, material models etc.

MESHING

Among the whole ship structure, the hull is given the most

importance and hence it is given the highest priority ogf most

accurate meshing.

First the hull part file is opened and to generate a mesh, go to

the Application and open Mechanica. In the window opened,

select structural and click ok

Assign materials and assign steel to the model. Now go to the

AutoGEM in the main menu and select control in that menu.

We will create dense meshing near sharp corners and with a

maximum size of 2 units whereas the whole length of the whole

ship is 800 units from fore to aft. Now we select the surfaces of

the ribs and the edge lines being made when the two surfaces

are intersecting at the bottom. Click ok and the surface will look

like the following.

The element budget is considered like this, First an auto mesh is

created and the results are computed. Next the degrees of

polynomial is increased to see if the results converge. If the

highest degree polynomial fails to do so, then the element size

is reduced and the calcucatins are performed again. This

procedure is carried out until the previous result converge

within acceptable range of accuray.